Diarylide yellow pigments are well known organic pigments and are commonly used in various types of printing inks. These pigments generally demonstrate good color strength, good transparency, adequate resistance to thermal degradation, and good light fastness. The diarylide yellow pigments are disazo pigments which are manufactured by tetrazotizing 3,3'-dichlorobenzidine to form the tetrazo salt. This tetrazo salt is then coupled with an acetoacetarylide to form the diarylide pigment. Depending on which acetoacetarylide is coupled into the 3,3'-dichlorobenzidine tetrazo salt, pigments are produced having different pigment characteristics such as color shade, resistance to bleeding in solvents, light fastness, and resistance to heat degradation.
The diarylide yellow pigments commonly used in the art are classified according to the acetoacetarylide coupling agent. AAA diarylide yellow is the reaction product of 3,3'-dichlorobenzidine tetrazotized and coupled into acetoacetanilide (AAA). AAMX diarylide yellow is the reaction product of 3,3'-dichlorobenzidine tetrazotized and coupled into acetoacet-2,4-xylidide (AAMX). AAOA diarylide yellow is the reaction product of 3,3'-dichlorobenzidine tetrazotized and coupled into acetoacet-o-anisidide (AAOA). AAOT diarylide yellow is the reaction product of 3,3'-dichlorobenzidine tetrazotized and coupled into acetoacet-o-toluidide (AAOT). HR diarylide yellow is the reaction product of 3,3'-dichlorobenzidine tetrazotized and coupled with acetoacet-2,5-dimethoxy-4-chloranilide.
Use of a different type of coupling agent, a pyrazolone derivative, gives redder pigments: 3,3'-dichloro benzidine tetrazotized and coupled into 3-methyl-1-phenyl-2-pyrazolin -5-one (PMP) gives PMP diarylide orange.
Redder diarylide pigments are made also by using another benzidine derivative, 3,3'-dimethoxy benzidine. When this is tetrazotized and coupled into AAA, dianisidine orange results; when coupled into PMP, dianisidine red results.
Diarylide yellow pigments are the primary organic yellow pigments produced in the United States. Of the diarylide yellow pigments, AAA diarylide yellow, also known as Pigment Yellow 12, is the primary pigment manufactured and used.
The initial step in the manufacture of diarylide yellow pigments is the tetrazotization of 3,3'-dichlorobenzidine. This is accomplished by reacting the 3,3'-dichlorobenzidine with hydrochloric acid and sodium nitrite to produce the tetrazonium salt, 3,3'-dichlorobenzidine tetrazonium chloride. ##STR1## The next step is the coupling of the tetrazonium salt with the acetoacetarylide to form a coupling product; in the case of AAA diarylide yellow, this would be acetoacetanilide. The coupling is typically accomplished by slowly adding the solution of the tetrazonium salt to a slurry of the acetoacetarylide in water. Coupling product is defined as the disazo reaction product of a tetrazo slat and an acetoacetarylide. The resulting reaction product of this coupling is the AAA diarylide yellow pigment. ##STR2##
The diarylide yellow pigment (i.e., the coupling product) must be recovered from a water-based pigment slurry when the reaction has been completed. This is typically done by the use of a pressurized filtration device such as a filter press to form a presscake. Typically the presscake comprises about 20% by weight of pigment and 80% by weight of water. Usually, this presscake is dried in an oven, etc., to reduce the water content and dry the pigment. The dried pigment is then dispersed into various types of printing ink vehicles. It is known in the art, however, that the drying of diarylide yellow pigments (that is, the drying of the pigment presscakes to produce a dry pigment) results in a decrease of the color strength and transparency of the pigment. Therefore, it is preferable in the art to form a flushed base by directly incorporating the wet presscake into an oily varnish. The flushed base typically will contain about 30% by weight pigment and about 70% by weight resins and oils. Flushing is typically accomplished by slowly feeding the presscake into a flushing vessel (flusher) containing a slow speed, high shear (e.g., sigma blade) mixer. The flushing process is disclosed in "Pigment Flushing" by T. Langstroth in Pigment Handbook, Temple C. Patton, Editor, 1973 (John Wiley & Sons, New York), Vol. III, pp 447-455, which is incorporated by reference. The oil, resins and pigment form one phase, while the water forms a separate phase which is decanted from the flusher. The flushed base remaining in the flusher contains little (e.g., 10%) water and comprises mainly the pigment, the resins, and the oil. The remaining water is removed under vacuum with heating.
There are several advantages associated with flushing the pigment into a resin base and then introducing the flushed base into a printing ink, rather than attempting to manufacture a printing ink by directly dispersing dry pigment into the ink. First of all, the flushing process is a low temperature process, so that the color strength and transparency characteristics of the pigment are not adversely affected as they would be if the pigment were dried; printing inks produced from flushed bases, as compared with printing inks produced from dried diarylide yellow pigment, tend to have increased color strength and transparency. In addition, when dried agglomerated pigments are dispersed in oils or resins, which are the vehicles of many printing inks, dispersing agents must be used on the pigments or in the vehicles in order to get a homogeneous dispersion in a reasonable period of time with minimum energy consumption; these dispersing agents often interfere with lithographic printing properties.
It is known in the art to treat diarylide yellow pigments with various compositions in order to improve the pigment characteristics. A general review of this subject is contained in "Surface Treatment of Organic Pigments for Printing Ink Applications," by B. G. Hays, in "American Ink Maker," Vol. 62, No. 6, (June, 1984), pp.28-50, which is incorporated by reference.
U.S. Pat. Nos. 3,775,148 and 3,776,749 disclose improved diarylide yellow pigment compositions made by adding one or more soluble dyestuffs to the pigment prior to coupling, during coupling, or after coupling. The pigments have improved strength and transparency.
U.S. Pat. Nos. 4,341,701 and 4,469,515 disclose methods of producing improved disazo pigments by a modified coupling technique. The pigments have improved strength and transparency. The process relates to adding a solution or suspension containing 0.1-50% by weight of the theoretically required total weight of one or more coupling components to a diazonium and/or tetrazonium salt of an aromatic or heterocyclic amine; this solution is then added to the other coupling components and the coupling reaction is completed.
U. K. Patent No. GB 2,108,143A discloses agents for enhancing the fluidity of pigment dispersions. The agents comprise a composition of disazo compounds containing a water-insoluble disazo compound in which one of the terminal aromatic groups carries a single substituted ammonium-acid salt group, such as dodecyl trimethyl ammonium or hexadecyl trimethyl ammonium para-sulfo-AAA yellow. Pigments are treated with the agent and nonaqueous pigment dispersions contain the agent.
U.S. Pat. No. 4,474,609 discloses improved mono azo pigment compositions made by adding di-, tri- or tetra-quaternary ammonium salts, such as permethylated tallow propylene diamine chloride, to mono azo pigment acid groups. The pigments have improved strength and recrystallization resistance.
It is known in the art to couple diarylide yellow pigments with tetrazotized 2,2'-disulfobenzidine and/or p-sulfo-acetoacetanilide to improve the pigment properties such as strength and transparency. However, the sulfonated AAA diarylide yellow pigments thus produced are very hydrophilic. These hydrophilic sulfonated disazo pigments, while having desirable improved properties, are not easily flushed into oily varnishes. When the flushing of presscakes of these sulfonated diarylide yellow pigments into oily bases is attempted, it is observed that the sulfonated diarylide yellow pigment compositions cannot be easily flushed into oily varnishes: the hydrophilic pigment prefers to remain in the water phase. Although most of the pigment eventually can be flushed into the oily phase, the water phase usually retains some amounts of diarylide yellow pigment which cannot be transferred, even with intense mixing, into the flushed base. Also, the resulting flushed base is usually extremely viscous, even after all water has been removed under vacuum.
Accordingly, this art needs improved sulfonated diarylide yellow pigment compositions which can be easily flushed into oily varnishes to form flushed bases.